unix: add Recvmmsg for linux

Add a Recvmmsg wrapper that exposes the recvmmsg(2) system call,
allowing callers to receive multiple messages from a socket in a
single syscall.

ps and oobs are parallel slices of per-message payload and control
buffers, with vlen derived from len(ps). Each message uses a single
buffer rather than a scatter-gather vector, which should be sufficient
for the typical recvmmsg use case. The return values ns, oobns,
recvflags, and from are parallel slices of length n (the number of
messages received).

This is a first step toward golang/go#45886.
The wrapper is useful as-is and lets callers experiment with batch
receive patterns before a poller-integrated net package API is
designed.

Author: nickgarlis

unix/linux/types.go

@@ -772,6 +772,8 @@ type PacketMreq C.struct_packet_mreq
 
 type Msghdr C.struct_msghdr
 
+type Mmsghdr C.struct_mmsghdr
+
 type Cmsghdr C.struct_cmsghdr
 
 type Inet4Pktinfo C.struct_in_pktinfo
@@ -826,6 +828,7 @@ const (
 	SizeofIPv6Mreq          = C.sizeof_struct_ipv6_mreq
 	SizeofPacketMreq        = C.sizeof_struct_packet_mreq
 	SizeofMsghdr            = C.sizeof_struct_msghdr
+	SizeofMmsghdr           = C.sizeof_struct_mmsghdr
 	SizeofCmsghdr           = C.sizeof_struct_cmsghdr
 	SizeofInet4Pktinfo      = C.sizeof_struct_in_pktinfo
 	SizeofInet6Pktinfo      = C.sizeof_struct_in6_pktinfo

unix/syscall_linux.go

@@ -1617,6 +1617,71 @@ func sendmsgN(fd int, iov []Iovec, oob []byte, ptr unsafe.Pointer, salen _Sockle
 	return n, nil
 }
 
+// Recvmmsg receives multiple messages from a socket using the recvmmsg system
+// call. ps holds the payload buffers and oobs the optional out-of-band control
+// buffers, one entry per message; vlen is derived from len(ps). Pass a nil or
+// zero-length oobs to receive no control data.
+//
+// The results are:
+//   - n is the number of messages received
+//   - ns[i] is the number of non-control bytes read into ps[i]
+//   - oobns[i] is the number of control bytes read into oobs[i]; interpret with [ParseSocketControlMessage]
+//   - recvflags[i] is the per-message flags returned by recvmmsg
+//   - from[i] is the sender address for message i, or nil for connected sockets
+func Recvmmsg(fd int, ps, oobs [][]byte, flags int) (n int, ns, oobns, recvflags []int, from []Sockaddr, err error) {
+	vlen := len(ps)
+	if vlen == 0 {
+		return 0, nil, nil, nil, nil, EINVAL
+	}
+	if len(oobs) > 0 && len(oobs) != vlen {
+		return 0, nil, nil, nil, nil, EINVAL
+	}
+
+	msghdrs := make([]Mmsghdr, vlen)
+	iovecs := make([]Iovec, vlen)
+	names := make([]byte, SizeofSockaddrAny*vlen)
+
+	for i := range vlen {
+		if len(ps[i]) > 0 {
+			iovecs[i].Base = &ps[i][0]
+			iovecs[i].SetLen(len(ps[i]))
+			msghdrs[i].Hdr.Iov = &iovecs[i]
+			msghdrs[i].Hdr.SetIovlen(1)
+		}
+		if i < len(oobs) && len(oobs[i]) > 0 {
+			msghdrs[i].Hdr.Control = &oobs[i][0]
+			msghdrs[i].Hdr.SetControllen(len(oobs[i]))
+		}
+		msghdrs[i].Hdr.Name = &names[i*SizeofSockaddrAny]
+		msghdrs[i].Hdr.Namelen = uint32(SizeofSockaddrAny)
+	}
+
+	n, err = recvmmsg(fd, &msghdrs[0], vlen, flags, nil)
+	if err != nil {
+		return 0, nil, nil, nil, nil, err
+	}
+
+	ns = make([]int, n)
+	oobns = make([]int, n)
+	recvflags = make([]int, n)
+	from = make([]Sockaddr, n)
+
+	for i := range n {
+		ns[i] = int(msghdrs[i].Len)
+		oobns[i] = int(msghdrs[i].Hdr.Controllen)
+		recvflags[i] = int(msghdrs[i].Hdr.Flags)
+		rsa := (*RawSockaddrAny)(unsafe.Pointer(&names[i*SizeofSockaddrAny]))
+		if rsa.Addr.Family != AF_UNSPEC {
+			from[i], err = anyToSockaddr(fd, rsa)
+			if err != nil {
+				return
+			}
+		}
+	}
+
+	return
+}
+
 // BindToDevice binds the socket associated with fd to device.
 func BindToDevice(fd int, device string) (err error) {
 	return SetsockoptString(fd, SOL_SOCKET, SO_BINDTODEVICE, device)

unix/syscall_linux_386.go

@@ -253,6 +253,14 @@ func sendmsg(s int, msg *Msghdr, flags int) (n int, err error) {
 	return
 }
 
+func recvmmsg(s int, mmsg *Mmsghdr, vlen int, flags int, timeout *Timespec) (n int, err error) {
+	n, e := socketcall(_RECVMMSG, uintptr(s), uintptr(unsafe.Pointer(mmsg)), uintptr(vlen), uintptr(flags), uintptr(unsafe.Pointer(timeout)), 0)
+	if e != 0 {
+		err = e
+	}
+	return
+}
+
 func Listen(s int, n int) (err error) {
 	_, e := socketcall(_LISTEN, uintptr(s), uintptr(n), 0, 0, 0, 0)
 	if e != 0 {

unix/syscall_linux_amd64.go

@@ -72,6 +72,7 @@ func Stat(path string, stat *Stat_t) (err error) {
 //sys	sendto(s int, buf []byte, flags int, to unsafe.Pointer, addrlen _Socklen) (err error)
 //sys	recvmsg(s int, msg *Msghdr, flags int) (n int, err error)
 //sys	sendmsg(s int, msg *Msghdr, flags int) (n int, err error)
+//sys	recvmmsg(s int, mmsg *Mmsghdr, vlen int, flags int, timeout *Timespec) (n int, err error)
 //sys	mmap(addr uintptr, length uintptr, prot int, flags int, fd int, offset int64) (xaddr uintptr, err error)
 
 //sys	futimesat(dirfd int, path string, times *[2]Timeval) (err error)

unix/syscall_linux_arm.go

@@ -41,6 +41,7 @@ func Seek(fd int, offset int64, whence int) (newoffset int64, err error) {
 //sysnb	socketpair(domain int, typ int, flags int, fd *[2]int32) (err error)
 //sys	recvmsg(s int, msg *Msghdr, flags int) (n int, err error)
 //sys	sendmsg(s int, msg *Msghdr, flags int) (n int, err error)
+//sys	recvmmsg(s int, mmsg *Mmsghdr, vlen int, flags int, timeout *Timespec) (n int, err error)
 
 // 64-bit file system and 32-bit uid calls
 // (16-bit uid calls are not always supported in newer kernels)

unix/syscall_linux_arm64.go

@@ -76,6 +76,7 @@ func Ustat(dev int, ubuf *Ustat_t) (err error) {
 //sys	sendto(s int, buf []byte, flags int, to unsafe.Pointer, addrlen _Socklen) (err error)
 //sys	recvmsg(s int, msg *Msghdr, flags int) (n int, err error)
 //sys	sendmsg(s int, msg *Msghdr, flags int) (n int, err error)
+//sys	recvmmsg(s int, mmsg *Mmsghdr, vlen int, flags int, timeout *Timespec) (n int, err error)
 //sys	mmap(addr uintptr, length uintptr, prot int, flags int, fd int, offset int64) (xaddr uintptr, err error)
 
 //sysnb	Gettimeofday(tv *Timeval) (err error)

unix/syscall_linux_loong64.go

@@ -108,6 +108,7 @@ func Ustat(dev int, ubuf *Ustat_t) (err error) {
 //sys	sendto(s int, buf []byte, flags int, to unsafe.Pointer, addrlen _Socklen) (err error)
 //sys	recvmsg(s int, msg *Msghdr, flags int) (n int, err error)
 //sys	sendmsg(s int, msg *Msghdr, flags int) (n int, err error)
+//sys	recvmmsg(s int, mmsg *Mmsghdr, vlen int, flags int, timeout *Timespec) (n int, err error)
 //sys	mmap(addr uintptr, length uintptr, prot int, flags int, fd int, offset int64) (xaddr uintptr, err error)
 
 //sysnb	Gettimeofday(tv *Timeval) (err error)

unix/syscall_linux_mips64x.go

@@ -56,6 +56,7 @@ func Select(nfd int, r *FdSet, w *FdSet, e *FdSet, timeout *Timeval) (n int, err
 //sys	sendto(s int, buf []byte, flags int, to unsafe.Pointer, addrlen _Socklen) (err error)
 //sys	recvmsg(s int, msg *Msghdr, flags int) (n int, err error)
 //sys	sendmsg(s int, msg *Msghdr, flags int) (n int, err error)
+//sys	recvmmsg(s int, mmsg *Mmsghdr, vlen int, flags int, timeout *Timespec) (n int, err error)
 //sys	mmap(addr uintptr, length uintptr, prot int, flags int, fd int, offset int64) (xaddr uintptr, err error)
 
 //sys	futimesat(dirfd int, path string, times *[2]Timeval) (err error)

unix/syscall_linux_mipsx.go

@@ -50,6 +50,7 @@ func Syscall9(trap, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr,
 //sys	sendto(s int, buf []byte, flags int, to unsafe.Pointer, addrlen _Socklen) (err error)
 //sys	recvmsg(s int, msg *Msghdr, flags int) (n int, err error)
 //sys	sendmsg(s int, msg *Msghdr, flags int) (n int, err error)
+//sys	recvmmsg(s int, mmsg *Mmsghdr, vlen int, flags int, timeout *Timespec) (n int, err error)
 
 //sys	Ioperm(from int, num int, on int) (err error)
 //sys	Iopl(level int) (err error)

unix/syscall_linux_ppc.go

@@ -53,6 +53,7 @@ import (
 //sys	sendto(s int, buf []byte, flags int, to unsafe.Pointer, addrlen _Socklen) (err error)
 //sys	recvmsg(s int, msg *Msghdr, flags int) (n int, err error)
 //sys	sendmsg(s int, msg *Msghdr, flags int) (n int, err error)
+//sys	recvmmsg(s int, mmsg *Mmsghdr, vlen int, flags int, timeout *Timespec) (n int, err error)
 
 //sys	futimesat(dirfd int, path string, times *[2]Timeval) (err error)
 //sysnb	Gettimeofday(tv *Timeval) (err error)